1. Field of the Invention
The present invention relates to a method and a system for continuously metering air into a flowing slurry composed of a dry cementitious material, an air entraining agent and a liquid--and for homogenizing said air into small cells to produce a uniform cellular product. For example, a plaster water slurry used in the metal casting industry to produce light density, aerated, precision plaster molds of high permeability for the production of castings of aluminum or magnesium. Or for a second example, a Portland cement water slurry used for insulating fill in the manufacturing of fire resistant safes, file cabinets, storage units etc. In still broader scope the field of the invention encompasses cellular concrete fills basically of Portland cement used in building construction or in geological technology which involves applications ranging from stabilization of highway embankments to grouting the annulus of relining pipe in sewer retrofit projects.
2. Description of the Prior Art
In the case of light density, cellular molds of plaster produced by companies making aluminum or magnesium castings, the generally accepted mold making method involves a batch process. The major gypsum manufacturers market a special plaster which incorporates a dry powdered surfactant blended into the plaster at the gypsum mill. To prepare a slurry of such plaster for pouring molds, a mixing pail is first charged with a given quantity of water, next a specified quantity of the special paster is added to the water. Following a brief soaking period, a portable (or fixed) motor driven high speed mixing shaft having a blade or disc at its tip is introduced into the mixing pail. The mold maker involved raises and lowers the rotating mixing shaft and disc thereby producing a vortex in the pail. As slurry is generated, the surfactant is activated and proceeds to aerate the slurry and to increase its volume particularly by drawing air into the vortex. After usually one to three minutes of mixing in this manner the body of the slurry has approximately doubled its volume and has become a homogeneous cellular material. When this prepared slurry is bucket poured into case molds or flasks it soon sets into a plaster solid which when dry forms the permeable production molds used in the casting process.
In the case of cellular concrete composed basically of Portland cement, a very different procedure is employed to prepare the slurry although it also is a batch process. Water is typically metered by volume into a mixing drum, then a measured weight of Portland cement is added. However, to aerate the slurry, a fixed quantity of pregenerated foam is added to the mixing drum and blended into the water/cement slurry as mixing by rotating agitators, proceeds. The pregenerated foam has been rendered stable and may be described as semi-solid as it issues or extrudes from the cylindrical nozzle of the foam generator. The foam generator is controlled by a timer and there has been previous calibration of the amount discharged timed to the tenth of a second. Thus the foam component of the mix is relatively accurately added to each batch as it is made. The agitator of blades of the drum type mixer continue to rotate during formation of the batch so that a homogeneous slurry is rapidly obtained. This rapid formation of slurry batches permits the process to function continuously because batches are discharged successively into a large hopper positioned below the mixing drum. The hopper in turn connects to a slurry pump. As batches are prepared and swiftly discharged into the slurry receiving hopper, the delivery pump is continuously pumping the slurry through a hose to the point of use. The rate of delivery or output is determined therefore by the rapidity with which slurry batches can be supplied to the large hopper of the delivery pump.
It can be understood from the above description that the state of the art for producing a cellular product of plaster or of Portland cement offers only imprecise quality control pertaining to certain properties of product, such as density or cellular structure. It is also evident that the production of aerated plaster molds used in production of aluminum or magnesium castings involves a tedious labor intensive process which is more art than technology.
In the production of Portland cement based cellular concrete, quality controls are not accurate because the foamed product is heavily dependent on the quality of the pregenerated foam and the accuracy with which the specified quantity of said foam is added to the batch in preparation. Moreover, in the mixing process pregenerated foam has a tendency to densify by some loss of cellular air and in pumping a foamed slurry through a hose additional loss of cellular air and further densification often occurs. In field practice, the density of the material deposited at the point of use is rarely more accurate than plus or minus three percent from one density check to another.